Recently, a field effect transistor comprising a metal gate electrode made of an alloy material such as a silicided metal has gathered attention. This field effect transistor comprising a metal gate electrode has an advantage that it reduces a compound capacity by eliminating depletion in a gate electrode and facilitates control of a Vth (threshold voltage) by controlling a work function.
Conventionally, there has been used a semiconductor device comprising an N-type field effect transistor (hereinafter, referred to as an “NMOS transistor”) and a P-type field effect transistor (hereinafter, referred to as a “PMOS transistor”) in which gate electrodes of these MOS transistors are combined to be a single line electrode. In this semiconductor device, the sections of the line electrode over the N-type and P-type areas formed within the semiconductor substrate correspond to the gate electrodes in each MOS transistor, respectively.
In the above semiconductor device, each of an NMOS transistor and a PMOS transistor may have a different work function of a constituent material for a gate electrode giving an optimal Vth. Thus, in this type of semiconductor device, it is necessary that a section corresponding to a gate electrode in each MOS transistor in one line electrode is made of a separate material and a work function of a constituent material for each gate electrode is controlled to individually optimize a Vth of each MOS transistor. Thus, there have been investigated technique for separately controlling work functions of gate electrode constituent materials in an NMOS transistor and a PMOS transistor.
A process for controlling a work function of a constituent material for a gate electrode is, for example, (1) forming a gate electrode in an NMOS transistor (hereinafter, referred to as a “gate electrode for an NMOS”) and a gate electrode in a PMOS transistor (hereinafter, referred to as a “gate electrode for a PMOS”) from materials containing mutually different elements, (2) forming a gate electrode for an NMOS and a gate electrode for a PMOS from materials containing the same elements with a different composition (atomic composition ratio) or (3) implanting a dopant element to both of gate electrode for an NMOS and gate electrode for a PMOS.
For example, as a method corresponding to the above (2) and (3), Japanese Laid-open Patent Publication No. 2005-129551 has disclosed a PMOS transistor comprising an Ni fully silicided electrode containing a P-type dopant with an Ni/(Ni+Si) composition ratio of 40 to 70 atomic % and an NMOS transistor comprising an Ni fully silicided electrode containing an N-type dopant with an Ni/(Ni+Si) composition ratio of 30 to 60 atomic %. In this semiconductor device, doses of a P-type and an N-type dopants to these gate electrodes and a composition ratio of Ni silicide on a silicon oxide gate insulating film are adjusted to be within optimum ranges. The reference has described that it can extend a modulation width of a work function and a Vth for a gate electrode for each MOS can be controlled to a desired value.